Add <InstallerApplicationIcon> to installers

This functionality is not yet supported on Unix, but it doesn't
fail if we put it there. It might work in the future though.

logo-small.icns and logo-small.ico are provided for Mac and Windows
respectively.

Change-Id: I53a58617fe128c32f32ca20ac660995b19657f02
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>